The present invention relates to a radiological imaging apparatus and radiological imaging method, and more particularly, to a radiological imaging apparatus and radiological imaging method ideally applicable to perform radiological imaging using X-ray computed tomography and positron emission computed tomography (hereinafter referred to as “PET”) and to perform radiological imaging using X-ray computed tomography and single photon emission computed tomography (hereinafter referred to as “SPECT”).
As radiological imaging methods using a human body as a test object, X-ray computed tomography, PET and SPECT, etc. are available. In PET and SPECT, a physical quantity is measured on an integral value (in a flying direction) of radiation emitted from a human body, and back projection of the integral value makes it possible to compute a physical quantity of each voxel in the human body and perform imaging. For such imaging, it is necessary to process an enormous amount of data. Rapid development of computer technology in recent years makes it possible to provide tomography of the human body with high speed and accuracy.
With PET and SPECT, it is possible to detect functions and metabolism at a level of molecular biology that cannot be detected by X-ray computed tomography and so on, and it is possible to provide a function image of the body of a medical examinee.
PET is a method comprising steps administering radiopharmaceutical (hereinafter referred to as “PET radiopharmaceutical”) including positron emitters (15O, 13N, 11C, 18F, etc., the half life is 2 to 110 minutes) to the examinee and examining locations in the body where more PET radiopharmaceutical are consumed. As an example of the PET radiopharmaceutical, 2-[F-18]fluoro-2-deoxy-D-glucose, 18FDG is available. Since 18FDG highly concentrates on tumor tissue due to carbohydratemetabolism, 18FDG is used for identifying a tumor. One positron is emitted from a positron emitter contained in the PET radiopharmaceutical concentrating on a specific point, and the positron couples with an electron of a neighboring cell to disappear and irradiates a pair of γ-rays having energy of 511 keV. These γ-rays are emitted in directions substantially opposite to each other (180°±0.6°). Detecting this pair of γ-rays (referred to as a γ-ray pair) using a radiation detector makes it possible to know between which radiation detectors the positron is emitted. Detecting many γ-ray pairs makes it possible to identify locations where more PET radiopharmaceutical are consumed. For example, as described above, since 18FDG highly concentrates on cancer cells having hyperactive carbohydrate metabolism. Thus, it is possible to discover cancer focuses using PET. The obtained data is converted to radiation density of each voxel using filtered back projection method, which is described IEEE Transactions on Nuclear Science, NS-21, pages 228 to 229, and the data contributes to imaging of locations of γ-rays (locations where a radionuclide concentrates, that is, a location of a cancer cell).
In SPECT, radiopharmaceutical (hereinafter referred to as “SPECT radiopharmaceutical”) containing a matter having a property of concentrating on a specific tumor or molecule and single photon emitters (99Tc, 67Ga, 201Tl, etc.) is administered to an examinee, and γ-rays emitted from the nuclides of the body are detected using a γ-ray detector. The energy of γ-rays emitted from the single photon emitters is around several 100 keV. Since the SPECT radiopharmaceutical concentrates on an area affected by cancer, it is possible to identify the cancer area. In the case of the SPECT as well, obtained data is converted to data of each voxel using a method such as filtered back projection. Besides, a transmission image is often taken in SPECT as well. 99Tc, 67Ga and 201Tl have a half life longer than that of radioisotopes used for the PET, for example, 6 hours to 3 days.
As described above, in the PET and SPECT, since a function image is obtained using internal metabolism, a part where radiopharmaceutical concentrates can be extracted with high contrast. However, it is not possible to know the position from adjacent organs. Thus, in recent years, the following technology has received attention: a conformation image as a tomographic image obtained by X-ray computed tomography is combined with a function image as a tomographic image obtained by the PET or SPECT to perform a high degree of diagnosis. As an example of the technology, technology described in JP-A-7-20245 is available.
In a radiological imaging apparatus of JP-A-7-20245, an X-ray computed tomography imaging apparatus and a PET imaging apparatus are placed in series, a bed where the examinee is laid down is moved horizontally, and examination is carried out on the examinee using the imaging apparatuses. Namely, an X-ray computed tomographic examination is carried out on the examinee using the X-ray computed tomography imaging apparatus, and then, a PET examination is performed on the examinee using the PET imaging apparatus. PET data and X-ray computed tomographic data which are the image data obtained by the two imaging apparatuses are combined to identify the focus location of the examinee.
JP-A-9-5441 describes a radiological imaging apparatus which also serves as a bed with an X-ray computed tomography imaging apparatus and a SPECT imaging apparatus placed in series. X-ray computed tomographic data and SPECT data which are the image data obtained by those imaging apparatuses are combined to identify the focus location of the examinee.
In the radiological imaging apparatuses of the above publications, the two different examinations are carried out with shifted positions. Thus, the examinations inevitably have a time lag.